Weeds present a tremendous problem to agricultural production throughout the world, and cause an estimated 10-12% loss of value for agricultural products in the United States, the most recent estimate being $20 billion annually (McWhorter, C. G. 1984! Weed Science, 32:850-855). Chemical pesticides are commonly used to control weeds in agricultural crops, but concern over environmental damage caused by these pesticides has recently elicited societal pressures to replace the chemical pesticides with alternative control methods. One area of active research in this area involves the use of plant pathogens, including both bacteria and fungi, to control pest plants in agricultural crops.
A major constraint to commercial development of a plant pathogen as a biological herbicide is selectivity. A pathogen that controls only one weed species in one type of crop does not have the same market potential as a pathogen that controls several important weed species in different types of crops.
Several methods are known in the art for biological control of weeds. As disclosed in U.S. Pat. No. 3,999,973, to Daniel et al., the anthracnose fungus Colletotrichum gloeosporioides has been used to control the weed northern jointvetch. Another strain of this fungus has been used to control winged waterpimrose. Colletotrichum malvarum has been used to control prickly sida. These three pathogens have been combined to control all three target weeds at once. In other experimental work the fungus Alternaria macrospora has been used to control spurred anoda (Anoda cristata), Weed Science, H. L. Walker, 1981, Vol. 29, pp. 505-507.
U.S. Pat. No. 4,390,360, dated Jun. 28, 1983, describes "Control of Sicklepod, Showy Crotalaria and Coffee Senna With A Fungal Pathogen" using the fungus Alternaria cassiae to produce typical weed lesions which kill or suppress the respective weeds. U.S. Pat. No. 4,419,120 dated Dec. 6, 1983, discloses "Control of Prickly Sida, Velvetleaf and Spurred Anoda with Fungal Pathogens" using the fungus Fusarium lateritium to kill or suppress the respective weeds. U.S. Pat. No. 4,715,881, dated Dec. 29, 1987, to Andersen, et al., details "Control of Eastern Black Nightshade with a Fungal Pathogen" using a strain of Colletotrichum coccodes which is pathogenic toward eastern black nightshade (Solanum ptycanthum). U.S. Pat. No. 4,718,935, dated Jan. 12, 1988 and U.S. Pat. No. 4,767,441, dated Aug. 30, 1988, describe a "Method For The Preparation of Mycoherbicide-Containing Pellets" characterized by alginate gel pellets containing living fungus capable of producing conidia when exposed to sufficient light and moisture. U.S. Pat. No. 4,724,147, dated Feb. 9, 1988, to James J. Marois, et al., and U.S. Pat. No. 4,818,530, dated Apr. 4, 1989, also to James J. Marois, et al., both detail the "Preparation of Pellets Containing Fungi for Control of Soilborne Diseases", in which fungi are first selected and grown for a time sufficient to produce inoculum. To prepare the pellets, the fungal propagules are harvested, homogenized, and diluted with sodium alginate solution. Pelletization is then accomplished by dropwise addition of the fungal propagule-alginate mixture into a solution of calcium chloride or calcium gluconate. The resulting alginate gel pellets containing living fungi can then be dried and used as inoculum.
U.S. Pat. No. 5,192,541, dated Mar. 9, 1993, to Steven D. Savage, et al., describes "Weed-Killing Xanthomonas campestris", in which novel microorganisms useful in controlling unwanted grasses and other weeds are discovered through a process which involves isolating plant pathogens from asymptomatic plants.